There are two different types of processes for manufacturing adipic acid. The conventional process for oxidizing cyclohexane (CH) to adipic acid involves two steps: the first step is to oxidize CH with oxygen to produce a mixture of cyclohexanone (CHO) and cyclohexanol (CHOL) at 150° C. in the presence of a cobalt or a manganese catalyst; the second step is to react the mixture of CHO and CHOL with nitric acid to yield adipic acid at 50-80° C. in the presence of a vanadium/copper catalyst. More recently, efforts have been made in the industry to develop a so-called “one-step process” to oxidize CH directly to adipic acid using oxygen in the presence of solvents, catalysts, and promoters.
One such one-step process is disclosed in U.S. Pat. No. 5,547,905 (Kulsrestha, et al.), which involves a catalyst preparation and activation to prepare adipic acid by oxidizing cyclohexane with air or oxygen. The catalyst consists of 70-99 wt % of a cobaltous salt and 1-30 wt % of a ferrous salt and is prepared in the presence of an initiator. The reaction is carried out at a pressure in the range of 1-70 kg/cm2 and a temperature in the range of 70°-150° C., for a period of 1-8 hours at a space velocity of 1-200 h−1. The reactor effluent typically contains unreacted cyclohexane, acetic acid (the solvent), water (a reaction product), adipic acid, succinic acid, glutaric acid, and the catalyst.
Recently, some developments have been made in removing and recycling the catalyst (mainly cobalt) from the product streams of such one-step manufacturing processes. The following references may be considered representative: International Publication WO 99/14178 (Rostami, et al.), International Publication WO 99/14179 (Dassel, et al.), International Publication WO 99/37599 (DeCoster, et al.), and International Publication WO 99/42430 (Dassel, et al.). In addition, these applications are related to the following U.S. Patents, which include many of the same inventors: U.S. Pat. No. 5,908,589 (DeCoster, et al.), U.S. Pat. No. 6,039,902 (Rostami, et al.), U.S. Pat. No. 6,103,933 (DeCoster, et al.), U.S. Pat. No. 6,129,875 (Dassel, et al.), and U.S. Pat. No. 6,218,573 (Vassiliou, et al.).
These interrelated documents all disclose methods of recycling a catalyst (cobalt) used in the oxidation of cyclohexane to adipic acid by a one-step process. Before the catalyst is precipitated from the reactor effluent, the major part of the adipic acid and other dibasic acid by-products is recovered, preferably by flash crystallization (under reduced temperature and pressure) followed by filtration.
Catalyst in the filtrate is partially precipitated and removed by reducing the water level in the mixture and/or subjecting the mixture to a temperature at which the catalyst precipitates. After the initial partial precipitation of the catalyst, the remaining mother liquor is subjected to a thermal treatment during which at least the major part of the acetic acid reactor solvent is removed, leaving behind molten dibasic acids, from which additional catalyst is precipitated and removed. The thermal treatment and catalyst removal can be carried out in two stages for better catalyst recovery. However, since the reaction products are recovered before the catalyst is removed, these methods cause the catalyst to co-precipitate or crystallize with the product, which makes the down stream product purification process more complicated and less efficient.
Other patents have discussed various methods for removing catalyst from oxidation reaction mixtures. For example, U.S. Pat. No. 5,880,313 (Zaima, et al.) describes a process in which an aromatic carboxylic acid product is crystallized and removed from the reaction liquid before a catalyst is precipitated. U.S. Pat. No. 5,756,837 (Costantini, et al.) describes a process for recycling a catalyst used in a direct oxidation reaction to convert cyclohexane to adipic acid. The catalyst is recycled by extracting the glutamic and succinic acids that are formed during the reaction. U.S. Pat. No. 4,254,283 (Mock) describes a process for preparing adipic acid from cyclohexanol and cyclohexanone by nitric acid oxidation. Glutamic and succinic acids are recovered as by-products. This process crystallizes the products from the reaction liquid after removing the nitric acid catalyst. Finally, U.S. Pat. No. 4,162,991 (Jones) describes a method for recovering a cobalt and bromide catalyst using a strongly basic anion exchange resin, followed by recovering the ions from the exchange resin by using a lower aliphatic monocarboxylic acid.
U.S. Pat. No. 3,959,449 (Shigayasu, et al.) describes a method for removing catalyst components including cobalt and manganese from a reaction mixture formed when an alkylbenzene is oxidized in a lower aliphatic mono-carboxylic acid as a solvent, in the presence of the catalyst. The catalyst is separated by forming an aqueous extract of the catalyst by stirring the reaction mixture with water in the presence of an oxygen-containing gas and a sulfur compound. The extract is then passed through a strongly acidic cation exchange resin to recover the catalyst.
U.S. Pat. No. 5,840,643 (Park, et al.) describes a method for removing a catalyst, including cobalt acetate tetrahydrate and manganese acetate tetrahydrate, from a reaction mixture produced by oxidizing pseudocumene to form trimellitic acid. The catalyst is removed from the reaction mixture before crystallization and distillation processes are performed. The method involves adding water to the reaction mixture in an amount ranging from zero to eleven times the amount of the reaction mixture. The diluted reaction mixture is then heated so that the diluted reaction mixture is in the liquid phase. The mixture is then passed through a cationic exchange resin to recover the catalyst.
U.S. Pat. No. 5,955,394 (Kelly) describes a method for separating a catalyst containing cobalt and manganese from a reaction mixture formed by oxidizing aromatic alkyls to produce aromatic carboxylic acids. The catalyst is removed from the reaction mixture before the reaction product is recovered. The method involves passing the reaction mixture through a strong acid cation exchange resin after heating the mixture to keep the aromatic acids in a dissolved state. The recovered catalyst is recycled to the reactor, and solvent can also be recovered and recycled.
None of the documents discussed above teaches the novel methods and apparatus for removing catalyst from a reaction mixture that are the subject of the present invention. Accordingly, there is a need to develop more efficient methods and apparatus for catalyst removal than those presently known for use in recovering catalysts from oxidation reaction mixtures.